(a) Field of the Invention
The present invention relates to a negative electrode of a lithium secondary battery, and more particularly to carbonaceous material for a negative electrode of a lithium secondary battery, the carbonaceous material being improved such that application thereof to the lithium secondary battery decreases irreversible capacity of a first cycle and increases discharge capacity of the lithium secondary battery. The present invention also relates to a lithium secondary battery made using the carbonaceous material.
(b) Description of the Related Art
With the proliferation in the use of portable electronic devices in recent times, coupled with advancements enhancing performance and enabling increasingly smaller sizes and weights for these devices, research is being actively pursued to improve the energy density of secondary batteries. One such type of secondary battery having high energy density characteristics is the lithium secondary battery Lithium secondary batteries utilize material that is able to undergo lithium ion intercalation and deintercalation respectively for a negative electrode and a positive electrode, and are filled with organic electrolyte or polymer electrolyte, which enable movement of lithium ions inside the battery (i.e., back to the negative electrode in the form of an ionic current).
The lithium secondary battery utilizes lithium metal as a negative electrode, and positive electrode material differing in oxidation potential from the lithium metal as a lithium ion carrier. Electrical energy is generated in the lithium secondary battery by processes of oxidation and reduction which take place when lithium ions undergo intercalation and deintercalation in the negative electrode and the positive electrode, respectively. With the use of lithium metal as the negative electrode in the lithium secondary battery, a serious problem of dendrite forming on a surface of the lithium metal results during charging and discharging. This may cause a short circuit, or more seriously may lead to the explosion of the battery.
To prevent such problems, carbonaceous material is now widely used for the active material of the negative electrode in what is known as a lithium ion secondary battery. Carbonaceous material is able to alternatingly either receive or supply lithium ions while maintaining its structural integrity and electrical properties. In the lithium ion secondary battery, lithium ions undergo intercalation and deintercalation between carbon layers. By the use of carbonaceous material for the negative electrode active material rather than directly using lithium metal, the reaction between active lithium and electrolyte is suppressed, and a short circuit caused by dendrite forming on the surface of the lithium metal is prevented.
However, in the lithium ion secondary battery using carbonaceous material as the negative electrode active material, since lithium ions are intercalated between the carbon layers, a capacity per gram is reduced by the amount of carbon present. As a result, although it is desirable to use a carbon material which can intercalate and deintercalate a maximum amount of lithium ions, a theoretical capacity of even graphite, which develops the most between the layers, is only 372 mAh/g. Further, an irreversible capacity of about 10% occurs by either a reaction between a surface of the graphite and an organic electrolyte, or a reaction between the lithium ions and cointercalated electrolytic material.
To increase the capacity of the carbon negative electrode, there is proposed the use of disordered carbon (amorphous hard carbon) which can intercalate lithium at other areas in addition to between the carbon layers. In the carbonaceous material, since there are many sites where lithium can be inserted, it is possible for part of the lithium to exist as metal clusters such that capacity is greatly increased. However, irreversible capacity is also increased with the use of amorphous carbon.